I'll pull the lower panels and look at the pipes. I've looked at everything between the motor and the inner firewall and no leaks there. It must be within the passenger compartment.
I'll check-out the aluminum stop-leak stuff. I saw some copper stop-leak at the car parts store also. Has anyone used it?
These products should be pretty easy to use. And by running the stuff only through the heater core, I can avoid exposing the interior of the engine to whatever damage they may cause. I figure that for about $10 and one-hour of my time, I should be able to plug the leak. Even if it only works for a couple months, it's not much of an investment in time or money.